1. Field of the Invention
The present invention relates to a liquid zoom lens for a portable terminal, and more particularly, to a liquid zoom lens that can provide an optical zoom function and an auto-focus function at the same time. In the liquid zoom lens, when a voltage is applied to the inside of a cylindrical body, the curvature of electrolyte and insulating liquid is varied. The electrolyte and the insulating liquid form a plurality of interfaces. Aspheric lenses are sequentially mounted to contact the interfaces or in the insulating liquid. The optical zoom function and the auto-focus function are achieved at the same time by the curvature variation of the electrolyte around the aspheric lens.
2. Description of the Related Art
Recently, cameras are built in portable terminals such as mobile phones or personal digital assistants (PDAs). Consumers give preference to the terminals with a built-in camera, which have high resolution and various functions. In these terminals with the built-in camera, lenses are attached to image pickup devices such as charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) elements. Thus, the terminals can take a picture of an object and can record data of the taken picture in a recording medium.
To provide the performance of mega-pixel cameras, the lens has to be designed to have sufficient resolution. In addition, the lens has to be designed to be larger than a real sensor size, considering the assembly tolerance.
A lens system is mounted on the portable terminal and used to take the image. In such a lens system, various kinds of aberrations are caused by incident light having different wavelengths when taking the image. The aberration causes the shape of the object to be distorted. Examples of the aberration include spherical aberration, astigmatism, and distortion. The lens system must be able to maximally prevent the aberration.
In the lens system, zoom is a function of providing a variable focal length. The zoom function is achieved by adjusting a distance between a front lens with a positive refractive index and a rear lens with a negative refractive index through their relative movement. To maximize the zoom function, a wide-angle lens or telephoto lens that can vary the focal length between an existing lens and an additional lens is separately attached to the camera. Therefore, the user can take the picture of the object at various viewing angles at one place without any movement.
The zoom can be classified into an optical zoom and a digital zoom. The optical zoom is to magnify the image of the object by a variable focal length while relatively moving an optical lens attached to the camera. The digital zoom is to magnify the image of the object within the CCD itself, as if the image is magnified using a graphic program such as Photoshop.
Unlike the optical zoom, because the digital zoom magnifies the image within the CCD, it needs no space for lens movement according to the variation of the focal length. Thus, the digital zoom is advantageous to the miniaturization and slim profile, but it cannot provide the high resolution.
On the other hand, because the optical zoom magnifies the image by the variation of the focal length between the lenses, it needs a space for the variation of the focal length. Due to a lens part and a lens barrel covering the lens part, the required space increases. However, the zoom lens provides the high resolution when zooming. Thus, in spite of the large volume of the terminal, the consumers prefer the optical zoom to the digital zoom.
As the currently available portable terminals tend to be smaller and slimmer, the digital zoom is preferred to the optical zoom because of the spatial limitation that can vary the focal length. Recently, the portable terminals that can provide the optical zoom function using the rear surface of the terminal are intermittently put on the market.
A technical structure of a conventional portable terminal that can provide the optical zoom is disclosed in Korean Patent Application No. 2003-3948, entitled “Lens Barrel Structure of Zoom Camera and Zoom Assembly”. In Korean Patent Application No. 2003-3948, an improved lens barrel structure of the optical zoom used in the digital camera is applied to the conventional portable camera. Therefore, the lens barrel and zoom assembly of the zoom camera is small-sized and easy to manufacture, and provides high resolution.
The conventional zoom assembly includes a front lens with a positive refraction index, a rear lens with a negative refractive index, an inner lens barrel, and an outer lens barrel. The inner lens barrel has a helicoid groove to guide helicoid locus motion of the front lens and the rear lens. The outer lens barrel is inserted outside the inner lens barrel and has an escape groove to guide the up/down motion of the front lens and the rear lens.
In such a conventional zoom camera, the inner lens barrel and the outer lens barrel are installed such that they are foldable in multiple stages on one side of the portable terminal. The inner lens barrel and the outer lens barrel are sequentially expanded by the driving of a motor within the terminal and thus the focal length is varied by the movement of the lenses, thereby achieving the optical zoom function. Consequently, there occurs the problem that a large space for the zoom function is occupied in the inside of the camera.
In addition, the motor installed in the camera has to be driven so as to move the inner and outer lens barrels with a plurality of lenses to the outside of the zoom camera. Power is dissipated in driving the motor and thus power dissipation of the battery increases.
To solve these problems, a liquid lens has been developed which occupies a small space and has low power dissipation in the portable terminal. The liquid lens achieves the zoom function using electrolyte and insulating liquid. A curvature of the electrolyte varies according to a voltage applied to a single lens barrel, and the insulating liquid forms an interface adjacent to the electrolyte.
A typical liquid lens that can provide the zoom function is disclosed in Korean Patent Laid-open Publication No. 2005-33308, entitled “Zoom camera using the liquid lens for mobile phone, control system and method thereof.” The conventional liquid lens will be described below with reference to FIG. 1.
FIG. 1 is a sectional view of a conventional liquid lens. Referring to FIG. 1, the conventional liquid lens includes a first lens group 310 having a first lens 311 with a positive refraction index and a second lens 312 with a negative refractive lens, a first liquid lens 300 in which a curvature radius of a contact surface between a conductive liquid and a non-conductive liquid is varied according to a zoom function control signal, a second lens group 330 having a third lens 331 with a positive refractive index and a fourth lens 332 with a negative refractive index, both sides of the third and fourth lenses 331 and 332 being aspheric, and an infrared filter 340 spaced apart from the second lens group 330 by a predetermined distance.
As illustrated in FIG. 2, the conventional liquid lens is based on an electrowetting phenomenon. The electrowetting phenomenon is caused because a surface tension of the interface is changed according to electric charges existing in the interface and thus its contact angle (α) is changed. A thin insulator is provided in the interface so as to increase the potential difference applied to the interface, and electric charges existing in the electrolyte are intended to move to the interface because of chemical properties.
At this point, when an external electric field is applied, the properties of the electric charges become stronger. Specifically, in a triple contact line (TCL) where the interfaces are overlapped, the concentration of the electric charges is greatly increased. Thus, the surface tension at the edge of the droplet is lowered.
Using the electrowetting phenomenon, it is possible to easily control micro droplets and micro particles within the liquid. Therefore, various products using the electrowetting phenomenon have been recently studied. The range of application includes liquid lenses, micro pumps, display devices, optical devices, and microelectromechanical systems (MEMS).
Compared with a mechanical lens, a liquid lens for auto-focus has a small size, low power dissipation, and high response speed.
In spite of these advantages, because the conventional liquid lens achieves the zoom function by the variation of the curvature radius between the conductive liquid and the non-conductive liquid of the individual liquid lenses within the single lens barrel to which the lens groups and the liquid lens are connected, the spatial limitation just like in the optical zoom lens using the multistage barrel can be solved, but the single liquid lens performs only the zoom function by the variation of the curvature between the internal liquids.
Furthermore, the conventional liquid lens has a complex structure. To achieve other functions (e.g., the auto-focus (A/F) function) as well as the zoom function by using the single liquid lens, another liquid lens acting as a front lens has to be provided. Consequently, the structure of the liquid lens becomes more complex.